This invention relates to medical diagnostic instruments, specifically such instruments for making measurements of magnetic fields in the human body, and, more specifically, to instruments wherein the location of the portion of the body being measured may be correlated directly with the biomagnetic measurements.
The human body produces various kinds of energy that may be used to monitor the status and health of the body. Perhaps the best known of these types of energy is heat. Most healthy persons have a body temperature of about 98.6.degree. F. A measured body temperature that is significantly higher usually indicates the presence of an infection of other deviation from normal good health. A simple medical instrument, the clinical thermometer, has long been available to measure body temperature.
Over 100 years ago, medical researchers learned that the body also produces electrical signals. Doctors today can recognize certain patterns of electrical signals that are indicative of good health, and other patterns that indicate disease or abnormality. The best known types of electrical signals are those from the heart and from the brain, and instruments have been developed that measure such signals. The electrocardiograph measures electrical signals associated with the operation of the heart, and the electroencephalograph measures the electrical signals associated with the brain. Such instruments have now become relatively common, and most hospitals have facilities wherein the electrical signals from the bodies of patients can be measured to determine certain types of possible disease or abnormality.
More recently, medical researchers have discovered that the body produces magnetic fields of a type completely different than the other types of energy emitted from the body. The research on correlating magnetic fields with various states of health, disease and abnormality is underway, but sufficient information is available to demonstrate that certain emitted magnetic fields are associated with conditions such as epilepsy and Alzheimer's disease. Present medical studies are investigating the nature of the normal and abnormal magnetic fields of the brain, and seeking to correlate those fields with the precise location in the brain from which they emanate. If it were known that a particular abnormality, such as Alzheimer's disease, were associated with an abnormal magnetic field produced at a particular location in the brain, then it might be possible to detect the abnormality at an early stage, while it was treatable, and then apply other medical knowledge to treat that precise portion of the brain. Magnetic studies of the brain therefore offer the potential for understanding and treating some of the most crippling diseases and conditions known.
The biomagnetometer is an instrument that has been developed for measuring magnetic fields produced by the body, particularly the brain. The biomagnetometer is a larger, more complex instrument than the medical instruments mentioned earlier, primarily because the magnetic fields produced by the body are very small and difficult to measure. Typically, the strength of the magnetic field produced by the brain is about 0.00000001 Gauss. By comparison, the strength of the earth's magnetic field is about 0.5 Gauss, or over a million times larger than the strength of the magnetic field of the brain. Most electrical equipment also produces magnetic fields, in many cases much larger than that of the earth. It is apparent that, unless special precautions are taken, it is not possible to make magnetic measurements of the human body because the external influences such as the earth's magnetism and nearby apparatus can completely mask the magnetic fields from the body.
The biomagnetometer is a medical instrument that includes a very sensitive detector of magnetic signals. The currently most widely used detector is a Superconducting QUantum Interference Device or SQUID, which is sufficiently sensitive to detect magnetic signals produced by the brain. (See, for example, U.S. Pat. Nos. 4,386,361 and 4,403,189, whose disclosures are incorporated by reference, for descriptions of two types of SQUIDs.) This detector and its associated equipment require special operating conditions such as a cryogenic dewar, and cannot be placed into the body or attached directly to the surface of the body.
The present biomagnetometer therefore provides a table upon which the patient lies, and a structure which places the SQUID in proximity with the head of the patient, as about 8 inches away. Special electronics is provided to filter out external effects such as the earth's magnetic field and the magnetic fields of nearby electrical instruments. (For a description of such a device, see U.S. Pat. Nos. 3,980,076 and 4,079,730, whose disclosures are herein incorporated by reference.) The patient and detector can also be placed into a magnetically quiet enclosure that shields the patient and the detector from the external magnetic fields. (For a description of such an enclosure, see U.S. Pat. No. 3,557,777, whose disclosure is herein incorporated by reference.) With these special precautions, medical researchers and doctors can now make accurate, reliable measurements of the magnetic fields produced by the brain, and are studying the relationship of these fields with diseases and abnormalities.
As discussed above, it is particularly important to be able to correlate the measured biomagnetic field with the exact location in the brain from which the field emanates. It is well established that certain physically identifiable locations in the brain are responsible for specific types of activities and functions. It is therefore important to correlate the measured biomagnetic field with the particular location in the brain which produces the field. Such a correlation is important to understanding the mechanism by which disease and disorder arise, and also to the treatment of the problem.
At the present time, there is no automatic, reliable, and accurate method for correlating the measured biomagnetic field with the precise location from which it emanates, within the head of the patient. The person whose biomagnetic fields are being measured may be asked to keep his head motionless during the course of the measurements, and his head position determined by physical measurements, photographs, or X-rays before or after the magnetic measurements. While this approach may have value in a few situations, in others it is nearly useless. The person may be asked to keep his head stationary to within 1 millimeter or so for several hours. For example, if the disease under study is epilepsy, many consecutive hours of observation may be required before an attack occurs. Movements of the person's head during the taking of data can invalidate any attempted correlation of the data with head position. That some data is invalid may be hard to detect, since the head movement may be brief and the patient may return his head to nearly the same initial location.
Alternatively, the person's head may be constrained to a relatively fixed position with a restraint system such as a frame and straps, and then photographed, X-rayed or physically measured at the beginning and end of the biomagnetic measurement session. This approach is likely to be uncomfortable for the patient, and may result in spurious signals that interfere with the biomagnetic measurements of interest. Even if the patient can keep his head stationary, the accuracy of correlation of the head position and the biomagnetic signal is not sufficiently great for some applications.
There is therefore a need for an apparatus and method for measuring biomagnetic fields and correlating those fields with the position of the patient's body, in a more exact manner than has been heretofore achieved. Preferably, such an approach would operate automatically to record head position in real time, so that the measured biomagnetic field could be correlated with the body position at the moment of measurement. Further, the measurement approach must not produce magnetic fields that are so large as to interfere with the principal function of the instrument, the measurement of very weak biomagnetic fields. This last requirement is particularly demanding, as most measurement devices having any electrical current flow produce magnetic fields of a magnitude that can interfere with the biomagnetic measurements. The present invention fulfills the need for such a biomagnetic measuring system, and further provides related advantages.